In the angry missive, Gilbert classified James' decision, made on a much-hyped ESPN prime time special, as a "shocking act of disloyalty, and he personally guaranteed that the Cavs will get a title before LeBron does.

Photos: LeBron James Heads to Miami Heat

"If you thought we were motivated before tonight to bring the hardware to Cleveland," the letter reads, " I can tell you that this shameful display of selfishness and betrayal by one of our very own has shifted our 'motivation' to previously unknown and previously never experienced levels."

In addition, Gilbert accused James of giving on the team during its first round playoff series.

"He quit," Gilbert said in a phone interview with the Associated Press after the decision was made public. "Not just in Game 5, but in Games 2, 4 and 6. Watch the tape. The Boston series was unlike anything in the history of sports for a superstar."